The use of printed circuit boards for a wide variety of applications is well known. Despite this variety, printed circuit boards share the common features of requiring formation of a metalization pattern on a substrate in a desired configuration. A common conventional technique for forming the metalization pattern on a surface (or surfaces) of a printed circuit board begins with a laminate comprised of a typically dielectric substrate clad with the metal, typically copper, on one or both sides. The metal cladding layer is applied by means of electroplating. Electroplating, however, requires use of chemicals which generate hazardous waste, including, for example spent sulfuric acid, drag out water (including copper sulfate and sulfuric acid), tin fluoboric, lead fluoboric and fluoboric acid, all of which must be properly disposed of. Following plating of the metal layer or layers, a subtractive process is used to remove the metal from the areas of the circuit board where it is not desired.
Many of these subtractive processes require use of a highly controlled wet chemistry process which may generate large quantities of undesirable hazardous waste, and which add a multiplicity of process steps to the formation of a circuit board. For the formation of a typical one-sided circuit board according to such a process, a masking step is first performed wherein a dry film photo-imagable resist is applied to the metal clad surface, typically by hot roll lamination. The resist is then imaged by placing a mylar imaging sheet (sometimes referred to as a "photomask") over the film, the imaging sheet having an image of the desired metalization patterned formed therein. The resist, as covered by the imaging sheet, is then exposed to UV light. After the mask is removed, a developer solution (illustratively 1% KOH or K.sub.2 CO.sub.3) is applied to the surface to dissolve and remove the resist in the areas of the circuit board where metalization is not desired. After developing, then, the surface of the circuit board includes resist covering the areas were metal is desired and the metal being exposed in the areas where metalization is not desired. The masked circuit board is then subjected to an etch step, wherein an etchant attacks and removes the copper or other metal in the unmasked areas. In an alternative process, the resist is patterned such that the copper is exposed in the areas where metalization is desired. The board is then solder plated, but only the exposed copper areas receive solder, since the resist prevents plating elsewhere. The resist is then removed by use of a solvent mixture including aliphatic amine and glycol ether in an inorganic base. The board is then placed in an etchant which etches the bare, unwanted copper, but not the solder plated copper. Regardless of which of these processes is used, a typical etchant for etching the exposed copper is a copper ammoniacal chloride solution (if the metal layer is copper). If a process was used in which resist remains on the board, the resist is then stripped from the remaining copper on the circuit board, using the solvent mixture described above. The typical mask and etch process for forming a conventional circuit board thus includes numerous process steps many of which generate hazardous waste which must be collected for proper disposal or reprocessing, thus adding expense to the process. Furthermore, in forming a double-sided circuit board, additional process steps, and thus a higher volume of chemicals, are required. In addition, the etching process often leads to undercutting of the foils forming the metalization pattern. Despite the fact that either resist or solder covers the desired foil, the etchant may still be able to partially attack the copper below.
A related subtractive method for forming printed circuit boards is so-called thin film processing. According to this process, an extremely thin coating (as opposed to the thick electroplated coating of the previous example) of copper or other metal is deposited to cover the entire surface of the dielectric substrate. Thin film processing typically uses either a sputtering process or vacuum deposition for depositing this extremely thin layer. Thus, the metal is deposited onto the substrate without need for plating solutions. However, once the thin layer of metal is in place on the substrate, standard masking and etching must still be performed in order to pattern the resulting metal layer, leading to the hazardous waste and undercutting problems previously mentioned. Moreover, because of the extremely thin coating applied to the substrate by the sputtering or vacuum deposition process, the board resulting from the mask and etch process must then be plated-up so that the metalization will be of sufficient thickness to achieve its desired function of conducting electricity with low resistivity. Such plating-up of a patterned circuit board may be performed either by electroplating (the preferred process), or by electroless deposition. In either case, the plating step that was omitted from the beginning of the process must be performed at the end of the process. Accordingly, thin film processing does not offer significant reductions either in process steps or in the generation of hazardous waste.
Other non-substractive techniques for forming printed circuit boards are also known. According to such techniques, wet chemistry is not required for masking and etching of an already-plated surface. Rather, thick conductive films are applied in the metalization pattern on the surface by means of silk screening. The conductive paths thus deposited by the thick film silk screen are then developed using either a high temperature process or a low temperature process. In the high temperature process, the thick film is formed of either silver palladium, gold platinum or gold palladium suspended in a butyl carbitol acetate solvent and including a binder of cellulose acetate. In the high temperature process, the thick film is exposed to a high temperature for the purpose for burning out the cellulose acetate binder that was used for placing the metal. Burn out of the binder, however, leaves voids in the conductive thick film. Accordingly, the metal remaining after the burn out process must be subjected to a sintering process to eliminate these voids. Because of the high temperature of both burn out and sintering, the high temperature process is limited in that it can be used only on dielectric substrates that can withstand the very high temperatures. Accordingly, the high temperature process is typically limited to ceramic substrates.
In the low temperature thick film process, the thick film is also applied to the substrate surface by means of silk screening. However, the thick film in this process includes a solvent of butyl carbitol acetate and naphtha including a thick film binder of phenolic resin and ether cellulose. In this process, the binder remains intact with the conductive medium and is not burned out as in the high temperature version. Thus, the low temperature version may be used on a wider variety of substrates. However, the low temperature thick film, primarily because of the continued presence of the binder, does not have the low resistivity typically desired in metal foils used for circuit boards. The range of applications for low temperature thick film circuit boards is thus limited by this low resistivity. While the two thick film processes described may generate less hazardous waste and include less process steps than the subtractive techniques, they are limited either by temperature considerations, or by undesirable electrical characteristics.